I've taken an Arduino mega plus a small SBC called a Pcduino (runs Ubuntu) and connected to a fingerprint sensor. You register your fingerprint and associate to an ID in the hr module for that employee.

A python script on the Pcduino connects to OpenERP via xmlrpc and logs the user in or out and displays a message in the GUI welcoming the employee.

My plan is to open source this but if I can get a handful of people who want one I can get some custom cases built so it's nicely packaged. If someone can help rewrite the Arduino fingerprint library for the Pcduino we can ditch the Arduino thus simplifying the design.